The persistence of O6-methylguanine produced by a single dose of N-methyl-N-nitrosourea (MNU) was determined in DNA of various murine tissues and compared with the location of tumours induced by MNU and related alkylating carcinogens in this species. A/J and C3HeB/FeJ mice received a single intravenous injection of MNU (10 mg/kg) and were killed at different time intervals ranging from 4 h to 7 days. The rate rate of loss of O6-methylguanine from brain DNA was considerably slower than from liver DNA; tumours have been found in both organs after administration of MNU and other alkylnitrosoureas. There was no difference in the rate of excision from cerebral DNA of A/J and C3HeB/FeJ mice, although these strains differ significantly in their susceptibility to the neurooncogenic effect of MNU and related carcinogens. Excision of O6-methylguanine from hepatic DNA was significantly slower in A/J than in C3HeB/FeJ mice; both strains habe been found to develop hepatic carcinomas following MNU administration. Seven days after the injection of 3H-MNU, O6-methylguanine concentrations were highest in brain and lung DNA, lowest in the liver, and intermediate in kidney, spleen, small intestine and stomach. The lung is a principal target organ for tumour induction by MNU and other carcinogens in mice; however, neural tumours are usually induced at a low incidence. The results obtained do not contradict the hypothesis that O6-alkylation of guanine in DNA is a critical event in the initiation of tumour induction by alkylating agents. However, the location of tumours produced in mice does not seem to depend solely on the formation and persistence of O6-alkylguanine in DNA.